Natural gas—often touted as an abundant, comparatively clean source of domestic energy—has come under intensifying public scrutiny in recent months, with federal regulators and reporters challenging some of the industry's rosy business projections.

Natural gas production has grown steadily in the United States since 2006, reaching new highs this year. But who are the leaders in this burgeoning field?

More than 14,000 oil-and-gas companies, many of them small businesses, were active in the United States in 2009, according to the Energy Information Administration. But multinational giants like Exxon Mobil and BP now produce much of the nation's gas. The 10 biggest drillers account for one-third of all production, data from the Natural Gas Supply Association and the EIA show. The 40 largest producers pump more than half of all domestic natural gas.

We've compiled a list of the top 10 drillers in the country, ranked by their daily natural gas production, and pulled together some key facts about their operations. Though there are other ways to measure these companies—revenue, market capitalization, reserves—industry experts say production numbers give the best snapshot of today's landscape and also separate drillers' gas operations from oil.

The list features both "integrated" oil-and-gas giants, such as Exxon Mobil, which refines and sells gasoline around the world, and "independents," such as Chesapeake Energy, which are primarily in oil and gas exploration and production. Though industry P.R. initiatives often emphasize independent mom-and-pop drillers, most of the companies on our list are Fortune 500 corporations.

Much of the growth in gas production has come from drilling into shale formations, which provided 23 percent of the nation's gas in 2010, according to the EIA. Our list shows how integrated behemoths have expanded into this area as production has become proven, sometimes by swallowing up independents that led the way. Last year, Exxon (No. 8 in 2009) bought XTO (No. 2 in 2009) to catapult to the top of the list. Also last year, Chevron (No. 9) bought Atlas Energy (No. 50 in 2009 and an early entrant into Pennsylvania's Marcellus Shale).

1. Exxon Mobil

The biggest natural gas producer is also the country's biggest oil company and one of the most profitable corporations in the world. Exxon has operations in every continent but Antarctica. Its oil and gas operations range across several states, from Pennsylvania to Colorado, and it also has wells in the Gulf of Mexico and off the California coast.

With the purchase of XTO, Exxon produces nearly 50 percent more gas than its closest competitor. Earlier this year, Exxon began running ads touting natural gas as a safe, clean source of domestic energy. About two-thirds of the company's domestic reserves are now in natural gas, with the rest in oil.

Average Daily Natural Gas Production: 3.9 billion cubic feet.

Revenue, 2010: $370 billion.

Reserves, 2010: 8.9 billion barrels of oil (2.3 billion in the U.S.), 2.1 billion barrels of bitumen (none in the U.S.), 681 million barrels of synthetic crude (none in the U.S.), 78.8 trillion cubic feet of natural gas (26.1 trillion in the U.S.).

Executive Compensation, 2010: Rex Tillerson, Exxon's chairman and CEO since 2006, received almost $29 million in total compensation.

2. Chesapeake Energy

Chesapeake calls itself the most active driller in the country, with operations in 15 states, from the Rockies to Texas to Pennsylvania. The company is a good example of how "independent" doesn't necessarily mean small. As of last year, the company owned an interest in 45,800 wells, of which 38,900 were primarily gas wells.

Chesapeake has built itself as a gas company, but it is increasingly looking for "liquids-rich plays," according to its annual report. Gas wells generally produce oil and other hydrocarbon liquids as well in varying amounts, depending on the geologic formation. With oil prices high and gas prices low, many companies are seeking more wells that are oil and liquids-rich, particularly in North Dakota, southern Texas and Pennsylvania.

Average Daily Natural Gas Production: 2.6 billion cubic feet.

Revenue, 2010: $9.4 billion.

Reserves, 2010: 14.3 trillion cubic feet of gas equivalent (10 percent of that is oil or other liquids, converted to the equivalent volume in gas).

Executive Compensation, 2010: Aubrey McClendon, the chairman and CEO, is also the company's founder. He has the unusual option of purchasing a small stake in every well the company drills. He received $21 million in total compensation.

3. Anadarko

Anadarko is one of the biggest independent oil and gas producers in the country, with exploration or production work in all major domestic drilling areas as well as in South America, Africa, Asia and New Zealand. The company was a minority owner in BP's Macondo well, which exploded last year, killing 11 people and spilling more than 200 million gallons of oil into the Gulf of Mexico.

Worldwide, natural gas makes up just over half of Anadarko's reserves, but 87 percent of the new wells it drilled in the United States last year were gas wells. Like many other companies, Anadarko is increasingly looking for oil and liquids-rich production this year.

Average Daily Natural Gas Production: 2.4 billion cubic feet.

Revenue, 2010: $11 billion.

Reserves, 2010: 749 million barrels of oil and condensate (458 million in the U.S.), 320 million barrels of natural gas liquids (307 million in the U.S.), 8.1 trillion cubic feet of gas, all in the United States.

Executive Compensation, 2010: James Hackett, the chairman and CEO, received $24 million in total compensation.

4. Devon Energy

Devon is an independent driller primarily active in the United States and Canada. The company is in the process of divesting operations in Angola and Brazil, its only holdings outside of North America.

More than 90 percent of Devon's U.S. reserves are in natural gas, with most of that lying in Texas' Barnett Shale. Like its peers, however, Devon says that this year it will focus on drilling in areas rich with oil and other liquids.

Average Daily Natural Gas Production: 2 billion cubic feet.

Revenue, 2010: $9.9 billion.

Reserves, 2010: 681 million barrels of oil (148 million in the U.S.), 479 million barrels of natural gas liquids (449 million in the U.S.), 10.3 trillion cubic feet of gas (9 trillion in the U.S.).

Executive Compensation, 2010: J. Larry Nichols, the chairman, received almost $19 million in total compensation. John Richels, president and CEO, received almost $18 million.

5. BP

Fortune lists BP as the fourth-largest corporation in the world. The company drills in 29 countries and sells its products in 70. While BP is headquartered in London, 42 percent of the company's assets are in the United States. BP reported a $3.7 billion loss last year after spending nearly $41 billion on cleaning up the Gulf oil spill and compensating those who were affected.

The company remains primarily an oil producer, with about 40 percent of its reserves in natural gas.

Executive Compensation, 2010: Chief Executive Robert Dudley received $1.7 million in total compensation.

6. Encana

Encana is one of the largest independent gas companies in the world, with operations mostly in the western United States and Canada, where it is based. The company has focused almost exclusively on gas.

Average Daily Natural Gas Production: 1.8 billion cubic feet.

Revenue, 2010: $8.9 billion.

Reserves, 2010: 93.3 million barrels of liquids (38.5 million in the U.S.), 13.8 trillion cubic feet of gas (7.5 trillion in the U.S.).

Executive Compensation, 2010: Randy Eresman, president and CEO, received $10 million in total compensation.

7. ConocoPhillips

ConocoPhillips is currently an integrated oil corporation, but it recently announced plans to split into two companies, one focused on refining, the other on production. The company has listed acquiring more shale reserves in North America among its top strategic goals over the past couple of years and drills in several Western states, as well as in Louisiana and Arkansas. It is exploring for shale gas in Poland and has operations in six continents.

Executive Compensation, 2010: James Mulva, chairman and CEO, received almost $18 million in total compensation. John Carrig, who retired as president in March, received more than $14 million.

8. Southwestern Energy Co.

Southwestern is another independent driller that focuses exclusively on natural gas. The company has operations in Arkansas, Texas, Oklahoma and Pennsylvania, with most of its production coming from the Fayetteville Shale formation underlying parts of Arkansas.

Executive Compensation, 2010: Steven Mueller, president and CEO, received $5.7 million in total compensation.

9. Chevron

Chevron is the second-largest oil company in the country, and the third-biggest company overall in terms of revenue. It has been building its gas reserves recently, most notably with the purchase of Atlas Energy, an active shale gas driller. Still, more than 60 percent of the company's worldwide reserves are in oil.

The majority of Chevron's oil and gas production comes overseas. Domestically, Chevron operates in seven states, including Pennsylvania, Texas and California, and in the Gulf of Mexico.

Average Daily Natural Gas Production:1.3 billion cubic feet.

Revenue, 2010: $198.2 billion.

Reserves, 2010: 6.5 billion barrels of oil and other liquids (1.3 billion in the U.S.), 24.3 trillion cubic feet of gas (2.5 trillion in the U.S.).

Executive Compensation, 2010: John Watson, chairman and CEO, received $16 million in total compensation.

10. Williams Energy

Williams is an independent producer focused largely on natural gas. It owns 13,900 miles of pipelines, which it says deliver 12 percent of the natural gas consumed in the United States. The company recently announced plans to separate its exploration and production activities from its other operations.

Williams has holdings in many of the major shale basins across the country, from Pennsylvania to North Dakota to Texas. The company also owns interests in several international companies.

Average Daily Natural Gas Production: 1.2 billion cubic feet.

Revenue, 2010: $9.6 billion.

Reserves, 2010: 4.3 trillion cubic feet equivalent (3 percent of that is oil or other liquids, converted to the equivalent volume in gas).

Executive Compensation, 2010: Alan Armstrong, president and CEO, received $2 million in total compensation.

Sources: The production numbers are from the Natural Gas Supply Association and reflect the average for the first half of 2011. Revenue figures are from the companies' 2010 annual reports and reflect total revenue from all sources, not just gas production. Revenue may include sales and other income and may not be adjusted for taxes. Reserves numbers are from the companies' annual reports. Bitumen and synthetic crude represent oil from Canadian tar sands or other unconventional reserves. The compensation information is from Forbes and Bloomberg Business Week.

Nicholas Kusnetz has written for The Nation, Miller-McCune, The New York Times and other publications. He is a graduate of UC Berkeley’s Graduate School of Journalism.